Waddell "A" Truss Bridge
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The Waddell "A" Truss Bridge is standardized truss bridge design that was first patented in 1893 by prolific civil engineer John Alexander Low Waddell. The design provided a simple low-cost, high-strength solution for use by railroads across the United States and Empire of Japan for short spans of around 100 ft (30.5 m). According to the Historic American Engineering Record (HAER), "beyond its role in the growth of railroad transportation, the "A" truss is perhaps most historically significant when viewed within the context of Waddell's career and the emergence of the American bridge fabrication industry into international marketing."[1]: 2
As a young civil engineer, J.A.L. Waddell spent the first decade after his graduation from Rensselaer Polytechnic Institute in the academic realm. He taught at his alma mater, authored numerous engineering papers, and eventually served as the Chair of Civil Engineering at the Imperial University at Tokyo between 1882-1886.[2]: 3–4 It was during this time in Japan that Waddell made many critical observations of the small-span bridges commonly in use on Japanese railroads at that point– almost all of which were designed by British engineers utilizing a riveted pony truss design.[1]: 3–4 As he would later recall in his 1898 publication De Pontibus:[3]
For a number of years the author was dissatisfied with all railroad bridges for spans between the superior limit of the plate-girder and a length of about one hundred and fifty feet, ordinary pin-connected through-Pratt trusses being too light and vibratory, and the riveted bridges as then built being clumsy, unscientific, and uneconomical.
— J.A.L. Waddell
To solve inadequate top-chord lateral bracing of less-rigid truss styles, Waddell's solution called for a 4-panel, triangular steel truss employing eyebars for the lower-chord and center-vertical elements, rigid beams for the rest of the structure, and heavy lateral bracing at the top point of the "A". The goal of this layout was to ease construction and minimize material costs, while still providing high rigidity and strength through foolproof pin-connected members.[1]: 5
